Process for clarifying oriented vinylidene chloride polymer films



Unite Carl P. Zupic, Midland, Mich., assignor to The Dow ChemicalCompany, Midland, Mich., a corporation of Delaware No Drawing.Application March 26, 1956 Serial No. 573,590

3 Claims. (Cl. 18-48) This invention relates to a novel process forclarifying thermoplastic films. More particularly it relates to aprocess for clarifying oriented plastic films having incomatible agentsincorporated therein.

,Plastic films, such as those produced from normally crystallinevinylidene chloride polymers have found wide acceptance as packagingmaterials. Many of such films and particularly those in which thepolymeric substance is composed predominantly of vinylidene chloridehave a tendency to stick to one another clue to electrostatic charges,on other cohesive forces resulting from their physical andchemicalmakeup, which has made the handling of such films extremely.diificult. Because of such tendencies, the full sales potential of thosefilms could not be realized. To overcome the ditficulties due tocohesion, two methods have been attempted. The first of such methodsinvolved dusting the surfaces of the film during production with solidmaterials. lit was found to be very diificult toapply the solidmaterials uniformly by mechanical means. The second method for reducingthe so hesive tendency of the film was to blend into the polymer, priorto fabrication, a small amount of an. incompatible, inert, inorganicsalt, suchas tetrasodium pyrophosphate, and to produce the film by theusual procedure, as by extrusion. The incompatible salt, although inextremely finely divided condition, was still capable of producing aroughened surface on the. film which resulted in greatly reducedcohesive tendency. The incompatible salt could be blended into theformulation simultaneously with the other common additives, such asstabilizers and plasticizers, and no added cost due to, extra proceduralsteps was involved; However, the addition of incompatible salts even infinely divided condition and. in. amounts as smallas 0.121 percentcaused a haziness, in the filmwhen the film was'oriented, reducing itsclarity. and transparency appreciably, It wouldthe desirable to haveaprocess for clarifying oriented. thermoplastic films. having anOpK'CiiY C ilJQ to presencetherein of incompatible agents, and theprovision ofsuch' a process is the principal object of this invention,

It is a further object to provide such a process whereby theanti-cohesive properties imparted to the film by the incompatible agentwill not be reduced.- i i The. above.- and related objects are.accomplished by m a s-9 aprocess where reat themwrlastic fili'nhavingincorporated therein an ineompatible; inert, inorganic compound toreduce the: cohesive. tendency thereof is subjected to a pressurecapable of squeezing the polymer into intimate continuous surfacecontact with the cohesive compound. When films having a haziness due tothe presence of such incompatible compounds are processed in thismanner, the resulting film is relatively clear and more transparent thanbefore treatment and its noncohesive quality is retained.

The polymers and copo-lymers capable of forming oriented, thin,transparent films are Well known. Among such polymers may be mentionedthose composed predominantly of vinylidene chloride with small butsignifis States Patent'G ice cant amounts of acopolymerizable'monoethylenically unsaturated monomer such as vinylchloride, vinyl acetate, or acrylonitrile. Such polymers represent apreferred class because of theirinherently strong cohesive tendencies inunmodified'condition, because ofthe unusually desirable properties offilms preparedfrom' them, and because of the case of orientation ofthosefilms.

Among the incompatible, inert, inorganic compounds thatimparta-nti-cohesive characteristics to orientedfilms may be mentionedthe inorganic phosphates. A preferred class of compounds for'use withthe normally crystalline vinylidene chloride polymers is exemplified bytetrasodiurn pyrophosphate, sodium tripolyphosphate, trisodiumphosphate, andsodium tetraphosphate'. In-anyca'se, the compound must beinert-to the'polymer so that it willnot'dissolve the polymer or bedissolvedin the polymer, degrade, or in any other-manneradversely-affect the properties of the film'dependent' upon the chemieal'nature ofthe polymer. The salts, such as copper andironsalts, which maydegrade certain polymers, such as normally crystalline vinylidenechloride polymers; are Well known in the" art and should be avoided.

it should be :apparent thatthe anti-cohesive agent must e in finelydivided condition foruse infilmproduction. When the particulate size i'sgreater' than a few-microns the particles causeloci ofpoortear'strengthinthe films. The agents must be solid-to impart' theroughness necessary to confer. anti cohesivequalities to the films.Additionally, the agentmust be capable of withstanding thenormal-temperatures encountered in the thermal fabrication of polymerswithout melting or decomposition.

The amounts of such agents that have'beenfound useful in conferringantircohesive qualitiesto plastic films ranges from 0.61 to l percentbyweight basedon the Weight of the polymer. Whensniallenamounts areemployed, the cohesive tendencies ofthe filmiiarenot're'duced andwhenlarger amounts are usedthierein is no beneficial result and thephysical'properties'of the= film sufieri Plastic films are. commonlypreparedby extruding the polymer. formulationin moltenlcondition throughasuitable die. To=achieve films: of normally crystalline polymers in asupercooled conditionfrom which they are orientable it is theusualzpractice. to :chill fthefreshlyextruded film by passage under lowtension through a bathof'nonsolvent at a relatively low temperature...Othermethods for: preparing filmsdncludethe casting; doctoring';-and"curing of solutions and latexesr ofithe; polymer. Toiachieve thesupercooled condition necessary to the orientation of cast films ofcrystalline polymers, it is then necessary to warm thevfilm to.a'temlperature capable of melting or dissolving the: individualcrysta-llites and then to chill the amorphous film'to a rel'atively lowtemperature.

When extrusionlmethods" areuto be used, the. agents maybBlDCOIPOI'fltBId by the. usual blending procedures used forincorporating. plasticizers and stabilizers into a polymer formulation.Those methods commonly consist of millingor mixing the ingredients forrather prolonged periods of time: When castingoperations are to beemployed inpreparingi the films, the anti-cohesive ingredients, ifinsolubleinthe dispersion medium, may berstirred into thesolutionworilatex prior to-casting.

Following, preparation and-.1 snper'cooling, the films are orientedby-rstrelt'chingr them beyond the-point of'ela'stic recovery. In sodoing, the crystallites that are formed have their major axes alignedprincipally in the direction of the applied stress which will beparallel to a major surface. During the stretching it has now been foundthat the polymer, being deformable under the stress, pulls away from thenon-deformable particles of the incompatible anti-blocking agent,leaving voids of various sizes and shapes surrounding the particles. Thehaziness that is apparent from such oriented films is believed to be dueto the light scattering caused by these polymer-void interfaces.

After orientation, the films are subjected to pressure and thermalconditions capable of filling those voids with polymer to therebydestroy thejpolymer-void interface. Although pressure may be employedalone at ordinary temperatures it has been found in such cases that therequired pressures are very high being of the order of 1000 pounds persquare inch or higher. Accordingly, it is preferred to combine thepressure treatment with an elevated temperature to reduce the resistanceto flow of the polymer. In addition, since the polymer is more plasticat the slightly elevated temperatures, the polymer will more readily andthoroughly conform to the rough surface of the particles ofanti-cohesive agent. It should be apparent that the maximum temperatureemployed must be well below the softening temperature of thecrystallites in the polymer so that the orientation is not substantiallydestroyed. For similar reasons it is desirable that the pressurizing beconducted with the film under enough tension to prevent destruction oralteration of the orientation due to film relaxation. This may be doneby applyingthe-pressure simultaneously over a relatively large area, asis possible with those reciprocal presses having large platens. If it isdesired to employ thermal means in conjunction with the pressure, theplatens may be heated by means of heat transfer fiuids. Similarly,pressure rolls may be used on continuous films maintained under tension.

The temperatures which have been found to be especially convenient whennormally crystalline vinylidene chloride polymers are used are fromabout 50 C. to 100 C., although higher or lower temperatures may beused. When using those temperatures the pressures that have been foundto be operable are from about 300 p. s. i. to 800 p. s. i. The pressuresand temperatures employed will vary to some extent depending upon thepolymer employed, the thickness of the film, the amount and nature ofanti-cohesive agent used, and the amount of orientation in the film.

The time required to clarify the films will vary with the pressures andtemperatures employed, the nature of the polymer and the thickness ofthe film. Within the ranges of temperatures and pressures given above ithas been found that when the films are of a thickness of 0.01 inch orless they should be pressurized for at least 30 minutes.

The operation and advantages of the process will be more apparent fromthe following illustrative examples wherein all parts are by weight.

Example 1 A film of a copolymer prepared from 85 percent by weight ofvinylidene chloride and 15 percent by Weight of vinyl chloride and inwhich was blended 0.2 percent by weight of tetrasodium pyrophosphate,was prepared by extruding the molten polymer formulation as a thin tubeinto a water bath at C. to supercool it. The tubular film was thenoriented radially and longitudinally, in conventional manner, and asingle thickness was found to have 9.9 percent haze as measured by thetransmission of the visible spectrum through the film using aspectrophotometer. The oriented film was placed in a press having heatedplatens and subjected to a pressure of 600 p. s. i. at 80 C- for 2hours. The resulting film was clear and was found to have 5.8 percenthaze.

For purpose of comparison a similar film was prepared but using notetrasodium pyrophosphate and was found to have a haze of 0.6%.

The cohesive tendencies of the films were checked using the followingprocedure. One layer of the film is attached to a variable angleinclined plane. A second sheet of the film is attached to a small flatweight of 0.07 pound with a contact area of 1 square inch. With thefilms in contact, the angle of the plane is increased until slidingoccurs. The larger the angle prior to sliding the greater is thecohesive tendency. With the oriented but unpressurized film containinganti-cohesive agent the maximum angle of repose was found to be 35 'Withthe oriented and pressurized film of this invention containinganti-cohesive agent the maximum angle was still 35. With the orientedfilm containing no anti-cohesive agent, the plane could be raised almostto without slippage.

Example 2 Extruded films prepared from 73 percent by Weight ofvinylidene chloride and 27 percent by weight of vinyl chloride andcontaining 1 percent tetrasodium pyrophosphate were supercooled andoriented. The films had 33.5 percent haze and a maximum angle of reposeof 42. After the films were subjected to a pressure of 600 p. s. i. at90 C. for 4 hours, they had 19.4 percent haze and still had a maximumangle of repose of 42.

A similar-film was prepared and oriented except that the tetrasodiumpyrophosphate was omitted. The film had 1.8 percent haze but the planecould be inclined to about 90 without slippage.

A secondary application of the process is to produce films of a givendegree of haziness for special effects in packaging. Thus, if it isdesired to prepare films having 25 percent haze, a film may first beprepared having a haze greater than 25 percent and the haze adjusted to25 percent using this process.

I claim:

1. A process for clarifying oriented thermoplastic polymer films of anormally crystalline vinylidene chloride polymer having an inherent hazedue to the presence in said filmof voids surrounding discrete particlesof an incompatible, inert, inorganic compound comprising subjecting saidfilm while under tension sufiicient to maintain said orientation to anapplied pressure of from 300 to 800 pounds per square inch at atemperature of from 30 to C., thereby to force said polymer intointimate engagement with said particles of said incompatible compound.

2. The process claimed in claim 1 wherein said incompatible compound istetrasodium pyrophosphate.

3. The process claimed in claim 1, wherein normally crystallinevinylidene chloride polymer is a copolymer of vinylidene chloride andvinyl chloride containing at least about 70 percent by weight vinylidenechloride.

References Cited in the file of this patent UNITED STATES PATENTS2,352,725 Markwood July 4, 1944 2,462,149 Webb Feb. 22, 1949 2,512,463Maier June 20, 1950 2,517,570 Irons Aug. 8, 1950 2,706,310 Eckler et al.Apr. 19, 1955

1. A PROCESS FOR CLARIFYING ORIENTED THERMOPLASTIC POLYMER FILMS OF ANORMALLY CRYSTALLINE VINYLIDENE CHLORIDE POLYMER HAVING AN INHERENT HAZEDUE TO THE PRESENCE IN SAID FILM OF VOIDS SURROUNDING DISCRETE PARTICLESOF AN INCOMPATIBLE, INERT, INORGANIC COMPOUND COMPRISING SUBJECTING SAIDFILM WHILE UNDER TENSION SUFFICIENT TO MAINTAIN SAID ORIENTATION TO ANAPPLIED PRESSURE OF FROM 300 TO 800 POUNDS PER SQUARE INCH AT ATEMPERATURE OF FROM 30* TO 100*C., THEREBY TO FORCE SAID POLYMER INTOINTIMATE ENGAGEMENT WITH SAID PARTICLES OF SAID INCOMPATIBLE COMPOUND.